Being that there is 4 red, 2 blue, 6 green, 8 white, that means there is 20 marbles, with 8 success, 8/20. Since the marble is not replaced, that drops it to 7/19 for the second pull. Multiply these together and you get 14.7%.

(8/20)*(7/19)=.147

.147*100=14.7